Streptococcus pneumoniae (S.p.), an important human pathogen causing pneumonia, is a major cause of morbidity and mortality worldwide. Its early mortality rate still remains the highest when compared to the pre-antibiotic era. Pneumolysin (PLY), a key cytolytic virulence factor, plays an important role in inducing acute alveolar hemorrhage and early lethality in severe S.p. infection. However, little is known about the underlying molecular mechanisms. Our Long-term objective is to fully understand the molecular mechanisms by which S.p. PLY-induced early lethality is caused and regulated in severe S.p. infections so that effective therapeutic strategy can be developed. Recently our published preliminary data indicate that tumor suppressor CYLD deficiency protects mice against S.p. PLY-induced alveolar hemorrhage and early lethality (Lim et al, Immunity, 2007). We also found that CYLD inhibits MKK3-p38-dependent type 1 plasminogen activator inhibitor (PAI1) expression in lung, thereby potentiating alveolar hemorrhage and early lethality. Of particular clinical significance is the direct evidence for the efficacy of the exogenous administration of PAI-1 in reducing alveolar hemorrhage and early lethality without serious adverse effects. These encouraging results have thus laid a solid foundation for us to further investigate the molecular mechanisms by which CYLD potentiates S.p. PLY-induced alveolar hemorrhage and early lethality via inhibition of MKK3-p38 -dependent PAI-1 expression (Hypothesis &Short-term Objective). Aim 1: Determine the molecular mechanism by which CYLD-deficiency protects mice against S.p. PLY-induced alveolar hemorrhage and early lethality via enhancing PAI-1 and the contribution of PAI-1 to the immune/inflammatory responses against S.p. Aim 2: Determine the molecular mechanism by which CYLD-deficiency protects mice against S.p. PLY-induced alveolar hemorrhage and early lethality via MKK3-p38-dependent up-regulation of PAI-1 expression. The proposed studies will provide novel insights into the molecular mechanisms underlying alveolar hemorrhage and lethality during early stage of severe S.p. infections and will lead to development of novel therapeutic agent for treating severe S.p. infections. Overall, the proposed studies will not only accelerate our scientific discovery and help further translate these discoveries from the basic science to clinical application, but will also contribute significantly to stimulating the economy by enabling hiring of additional professional and technical staff with needed expertise's and skills.